Hand-held self-adhesive-label applicators

ABSTRACT

An adjustable facet print mechanism includes a plurality of print-facet carrying bands, each having a plurality of printfacets, a corresponding plurality of first wheels arranged on a common axis, each wheel having an internal array of projections and carrying a series of characters corresponding to those of a related print band, a corresponding plurality of second wheels arranged on a common axis parallel to the axis of the first wheels and each serving in part to support one band, means serving additionally to support the bands and providing backing surfaces for the print-facets of the bands operative at any given time, means drivingly connecting corresponding first and second wheels, a threaded member rotatable on the common axis of the first wheels and lying within these wheels, a nut member carried by the threaded member and having an array of projections on its periphery, a rotatable spider member having a plurality of arms interdigitating with at least some of the projections of the nut member, means for preventing rotation of the spider member, and a single actuating member operative selectively to drive the threaded member, with the means for preventing rotation of the spider operative, whereby to move the nut member to a position whereat the projections thereof mesh with the internal projections of a selected first wheel and to drive the screwed member, with the means for preventing rotation of the spider inoperative, whereby to rotate the nut member and spider together to index the selected first wheel and associated print band to a required operative print facet.

United States Patent 1 1 Nicholson 1 1 HAND-HELD SELF-ADHESlVE-LABEL APPLICATORS [75] Inventor: Brian Nicholson, Boston, England [73] Assignee: Norprint Limited, England [22] Filed: July 10, 1973 [21] App]. No.: 378,031

Primary ExaminerEdgar S. Burr Assistant Examiner-Edward M. Coven Attorney, Agent, or Firm-McGlew and Tuttle [57] ABSTRACT An adjustable facet print mechanism includes a plural- 1 June 17, 1975 ity of print-facet carrying bands, each having a plurality of print-facets, a corresponding plurality of first wheels arranged on a common axis, each wheel having an internal array of projections and carrying a series of characters corresponding to those of a related print band, a corresponding plurality of second wheels arranged on a common axis parallel to the axis of the first wheels and each serving in part to support one band, means serving additionally to support the bands and providing backing surfaces for the print-facets of the bands operative at any given time, means drivingly connecting corresponding first and second wheels, a threaded member rotatable on the common axis of the first wheels and lying within these wheels, a nut member carried by the threaded member and having an array of projections on its periphery, a rotatable spider member having a plurality of arms interdigitating with at least some of the projections of the nut member, means for preventing rotation of the spider member, and a single actuating member operative selectively to drive the threaded member, with the means for preventing rotation of the spider operative, whereby to move the nut member to a position whereat the projections thereof mesh with the internal projections of a selected first wheel and to drive the screwed member, with the means for preventing rotation of the spider inoperative, whereby to rotate the nut member and spider together to index the selected first wheel and associated print band to a required operative print facet.

12 Claims, 20 Drawing Figures SHEET PATENTEDJUN 1 7 m5 Svm mm QM FQ K E X Em mm PATENTEIJJUN 17 m5 SHEET m w WN E ma mm vs Q3 PATENTEDJUN 1 7 I975 SHEET mm Q gut am 3 a e 9 5 94 PATENTEIJJUN 17 SHEET 5 PATENTEDJUN 17 ms 1- 889,594

SHEET 6 54 na/o. 61

FIG. I314. FIG. I38.

1 HAND-HELD SELF-ADHESIVE-LABEL APPLICATORS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to adjustablefacet print mechanisms, for example for use in hand-held self-adhesivelabel applicators.

2. Description of the Prior Art Adjustable-facet print mechanisms have previously been proposed in which print bands can be adjusted manually to give a required print-out. Such mechanisms are commonly employed in date stamps, but such date stamps are rarely required to give a good print impression and furthermore, because the impression is changed only relatively infrequently, the fact that the users fingers may become soiled by ink from the print bands is of relatively little account.

More recently similar, but generally more elaborate, print mechanisms have been employed in marking devices for use in supermarkets and other stores and the requirements of use are far more onerous than in the case of data stamps. The print impression must be completely clear and, because the operator is likely to change the print-out fairly frequently, it is clearly desirable that direct manual contact with the print bands should be avoided. Again, it is important that the operator should know with absolute certainty the print-out after any resetting operation and to avoid the risk of confusion it is not desirable to rely upon the negative impression which can be viewed by the user at the print-out position of the print-bands or wheels. It follows that the mechanism should include a clearly legible reproduction in positive characters of the print facets operative at any given time.

Furthermore it is a requirement of practice that changes of the print-out should require only a simple operation by the user and that the use of a multiplicity of different control members should be avoided. Again, the use of a number of control members is undesirable because the amount of space taken gives rise to diffrculties in providing a compact mechanism.

It is therefore an object of the present invention to provide an adjustable-facet print mechanism which can adequately meet the demands of practice.

SUMMARY OF THE INVENTION According to the present invention there is provided, in an adjustable facet print mechanism, a plurality of print-facet carrying bands, each having a plurality of print-facets, a corresponding plurality of first wheels arranged on a common axis, each wheel having an internal array of projections and carrying a series of characters corresponding to those of a related print band, a corresponding plurality of second wheels arranged on a common axis parallel to the axis of the first wheels and each serving in part to support one band, means serving additionally to support the bands and providing backing surfaces for the print-facets of the bands operative at any given time, means drivingly connecting corresponding first and second wheels, a threaded member rotatable on the common axis of the first wheels and lying within these wheels, a nut member carried by the threaded member and having an array of projections on its periphery, a rotatable spider member having a plurality of arms interdigitating with at least some of the projections of the nut member, means for preventing rotation of the spider member, and a single actuating member operative selectively to drive the screwed member, with the means for preventing rotation of the spider operative, whereby to move the nut member to a position whereat the projections thereof mesh with the internal projections of a selected first wheel and to drive the screwed member, with the means for preventing rotation of the spider inoperative, whereby to rotate the nut member and spider together to index the selected first wheel and associated print band to a required operative print facet.

BRIEF DESCRIPTION OF THE DRAWINGS One embodiment of an adjustable-facet print mechanism in accordance with the present invention will now be described by way of example, with reference to the accompanying diagrammatic drawings, in which:

FIGS. 1A and 1B are together a longitudinal section of the embodiment in accordance with the invention;

FIG. 2A is an end elevation showing part of the mechanism of FIG. 1 and FIG. 2B is a side elevation of the same part;

FIG. 3 is a section on the line llIIIl of FIG. I;

FIG. 4 is a fragmentary plan view of a read-out facility of the mechanism;

FIGS. 5A and 5B are respectively a section of one part of a detail and an end elevation of the same detail;

FIG. 6A is a longitudinal section ofa clutch member of the lead screw assembly and FIG. 6B is an end view of the clutch member;

FIG. 7 is a longitudinal section of a nut member which is engaged on the lead screw assembly;

FIG. 8 is an end view of the nut member of FIG. 7; FIG. 9 is a side elevation of the nut member of FIG.

FIG. 10 is a longitudinal section of a spider member forming a part of the mechanism;

FIG. 11 is an end elevation of the spider viewed in the direction of the arrow 11 of FIG. 10;

FIG. 12 is an end elevation in the direction of the arrow 12 of FIG. I0;

FIGS. 13A and 13B are respectively an end elevation and side elevation of a detail; and

FIGS. 14A and 14B are respectively a longitudinal section and an end elevation of a control knob of the mechanism.

Referring now to the drawings, and particularly to FIGS. IA and "3, an adjustable-facet print mechanism in accordance with the invention is intended for incorporation in a self-adhesive label applicator, for example as disclosed in our co'pending application Ser. No. 283,212 and comprises a framework or casing having end plates 10, 11 between which the working parts of the mechanism are assembled and secured. The end plate 10 has a plane external surface whilst its internal surface is formed with a relatively smaller annular boss 12 and a relatively larger annular boss 14, the latter being stepped at its inner end 16.

The end plate 11 has an external annular boss 18 coaxial with the boss 14 and a single internal annular boss 20 which is of the same size as and lies directly opposite and co-axial with the boss 12 of the end plate 10. The end plates 10, 11 are joined by a peripheral structure indicated at 22 which has a transverse opening or slot 24 (see also FIGS. 3 and 4) for a read-out facility to be described in detail hereinafter. The end plates I0, 11 and structure 22 will normally form parts of the external casing of apparatus (not otherwise shown) with which the mechanism is associated.

The boss 18 of the end plate 11 is partially covered by a control knob 30 (see FIGS. 1 and 14A. 148) which is of stepped cylindrical form including a smaller diameter portion 31 and a larger diameter portion 31A. both portions having flne teeth 31B and 31C respectively on their peripheries to provide a good grip. A sleeve 32 integral with the portion 31 extends coaxially within both portions 31 and 31A. but stops slightly short of the end plane of the portion 31A. The internal surface of the sleeve includes a plain cylindrical portion 32A, a step 328, a frusto-conical portion 32C. and a further plain cylindrical portion 32D. The purpose of these portions of the internal surface of the sleeve 32 will appear hereinafter. The knob portions 31 and 31A are interconnected by a generally annular portion 33 and this carries an integral annular flange 33A the internal surface 338 of which is octagonal. The flange 33A lies intermediate the portion 31A and the sleeve 32.

The octagonal surface 33B of the flange 33A is arranged to engage at the corners of the octagon with a pair of elongate projections 34 (FIGS. 6A and 68) forming parts of a clutch member 35. Each projection 34 extends parallel to the disc-like base of the clutch member 35 and is connected thereto by a stem 34A. As will be apparent from FIG. 68 each of the elongate members 34 has a rounded tip 348 and this facilitates overriding of the octagonal inner surface 338 of the flange 33A when the torque imposed is excessive. the members 34 flexing under such conditions. The disclike base of the clutch 35 carries an axial spigot 36 having an annular flange 37 at its free end and this engages within the sleeve 32 of the knob 30. The step 328 of the sleeve serves to retain the flange 37 of the spigot 36. The clutch member 35 also carries on its main disc-like base an array of thirteen equallyspaced chamfered dogs or teeth 38 which lie on the opposite side of the base to the elongate projections shown in FIG. 68. Each of the dogs or teeth is of sector section.

The periphery of the disc like base of the clutch member 35 is substantially in contact with a bush 188 (FIG. 1A) within a flange 18A of the boss 18.

The clutch member 35, as can be seen from FIG. 1A is juxtaposed to a shaft 40A which carries a lead screw 40 having an external screw-thread with a three-start acme thread 41 of coarse pitch along the major part of its length and. at the end of the shaft 40A remote from the clutch member 35, there is an annular groove 44 arranged to carry a retaining circlip or washer 45. A similar groove 44A and spring washer 45A is provided adjacent clutch member 35. To ensure correct assem' bly one thread is of greater width than the other two and the nut member shown in FIGS. 1, 7, 8 and 9 is formed with female threads of complementary form. A flange 46 extends radially from the lead screw 40 and carries at its outer periphery an integral sleeve 47 extending co-axially and inwardly and this sleeve, in turn, carries a further radially-extending flange 48 which lies radially outwardly of one end of the screw thread 41.

Recesses 40 (FIG. SA) in the end flange 46 serve to engage the dogs 38 of the clutch member 35 and also dogs 52 of a skeleton sleeve or spider 54 (see FIGS. 1. l and 11) which extends co-axially with the lead screw 40. The individual members 53 (thirteen in number) ofthis spider 54 are clearly shown in FIGS. and

11, and the end portions thereof remote from the dogs 52 together carry a flange-like member 55 lying radially outwardly of the end of the lead screw 40 remote from clutch member. The member 55 has an array of dogs 56 arranged to engage corresponding dogs 58 fast with the inner surface of the end plate 10. The flange 55 also has an annular recess 60 which accommodates one end ofa spiral spring 63 the other. smaller, end of which abuts the washer 45 engaged on the reduced diameter portion 42 of the lead screw 40. Finally, the flange 55 includes a cylindrical co-axial surface 61 and this provides support for the spider 54 by being a sliding fit in a bush 61A mounted within the boss 14 of the end plate 10.

The screw-threaded portion of the lead screw 40 carries a nut member 62 (see FIGS. 1, 7, 8 and 9) which is guided along the individual legs of the spider 54 by an array of teeth or dogs 64 which also serve to engage corresponding internal teeth 66 of a selected one of a plurality of read-out wheels 70 (first wheels) which are held between the flange 48 of the lead screw 40 and the stepped end portion 16 of the boss 14 of the end plate 10. The regularly-spaced internal teeth 66 of each read-out wheel 70 serve to support the wheel on the spider 54 and each wheel has an axially-extending integral ring 74 axially locating on the immediately adjacent read-out wheel 70 or the face 78 of the flange 48 in the case of the end one of the wheels, the axial extent of the rings 74 being such that a gap is left between each of the read-out wheels 70. An end face 76 of the other end wheel 70 is engaged by a ring abutting the bush 61A carried within the boss 14.

To prevent the nut 62 from jamming at the ends of its travel along the thread 41, stop surfaces 62A (FIGS. 8 and 9) are formed at each end of each thread helix of the nut member and these stop surfaces co-act with corresponding stop surfaces (not shown) at the flange end of the thread 41 and stop surfaces 408 on a spacer or collar 65 (FIGS. 1A, 13A and 138). The collar 65 lies between the washer 45 and the step in the lead screw 40 and serves primarily as a spacer.

As will be apparent from FIG. 3, the read-out wheels 70 are deeply crenellated to form a plurality of individual teeth 71 with the read-out facets 79 at the outer periphery of each tooth. It will be apparent from FIG. 1 that the ratio of the axial thickness of the read-out wheels 70 is 3: 4: 6 and the same ratio applies to the print bands and print wheels to be described in detail hereinafter. The combination of wheels can be varied to accommodate print bands 81 of corresponding widths which incorporate different characters. This facility is of particular importance where letters other than latin and numerals other than arabic are to be used.

The teeth 71 of each wheel 70 are in constant mesh with thin, lobe-shaped, projections provided on the periphery of the corresponding one of a plurality of print-band carrier wheels 82 (second wheels), the print bands 81 engaging on a peripheral part of each carrier wheel below the radial level of the projections 80. Each carrier wheel has a plurality of peripheral notches 83 spaced by a distance equal to the spacing of the character-carrying segments of the print bands. A corresponding number of radially-extending grooves 85 of semicircular section is provided on one side face of each wheel. A continuous annular groove 85A (FIG. 1B) is provided on the other side face of each wheel. The

print band carrier wheels 82 are rotatably mounted on a shaft 86 through the intermediary of a sleeve 89, the shaft being journalled by portions 88 in the bosses 12, of the end plates 10, 11 respectively. The shaft and sleeve have complementary tapers to assist moulding. The facility of providing a bearing relationship between the boss 20 and the portions 88 assists assembly. This is further assisted by two spigots (not shown) at the ends of the sleeve remote from a plate-like member 87 carrying the sleeve and complementary apertures (not shown) in a plate member.

The number of and thickness of the print-band car rier wheels 82 corresponds with the number and thickness of read-out wheels 70, and these print-band carrier wheels are spaced by thin plate or shim members 120 (spacers) which extend to and engage over a fixed bar 92 which carries further carrier wheels 90, (third wheels see FIG. 3) each print band 81 being trained around a corresponding wheel 90 and spaced from the adjacent print bands by portions of the members 120. The bar 92 has a lower flat surface, two inclined surfaces extending upwardly away from the flat surface at the same angles as the corresponding sides of a pentagon, two vertical, parallel opposite sides and a semicircular upper surface interconnecting the parallel opposite sides. The bar 92 carries the wheels 90 which are of generally pentagonal form, each face of the pentagon having a recess 91 of semi circular form in each, otherwise plane face. Each wheel 90 includes an integral generally annular flange 90A and a generally pentagonal aperture 93 which co-operates with the bar 92 and has a shallow recess 94 in each side. As any given wheel 90 rotates on the bar 92 resistance is generated by the shape of the bar and the aperture 93, but when the generally plane faces correspond, an essentially flat support is given to the operative print facet of the corresponding print band 81. For comparatively wide print bands 81, it is preferable to reduce the width of the pentagonal surface of the corresponding wheel 90 since otherwise the stiffness of the band makes turning difficult. The non-pentagonal portion is made cylindrical.

Each spacer 120 as shown in FIGS. 2A and 2B comprises a generally plane annular portion 121 and, as shown, a depending portion 122 having an elongate aperture 123 is arranged to receive the bar 92. The portion 121 receives the sleeve 89 in the aperture thereof. Upstanding dimples 123A, 123B are provided on respective faces by pressing to provide the spacing and biasing function of the member 20. When assembled the dimples 123A of each spacer 120 engage in the groove 87 of an adjacent wheel 82 and the dimples 1238 provide indexing stops in conjunction with the radial grooves 85 in the other adjacent wheel. A spring 125 at one end of the stack of wheels serves to bias them against the dimples of the interposed spacers.

The operation will now be described primarily with reference to FIGS. 1 and 3. At any given time the position of the nut member 62 along the length of the lead screw 40 is readily apparent by inspection through the slot 24 (FIGS. 3 and 4) because it is made of a color which contrasts with the color of at least the spider 54 and lead screw 40. Assuming that the nut member 62 is at the desired position it is merely necessary to rotate the knob which serves to index the corresponding read-out wheel 70 and print band carrier wheels 82 and 90 until the desired new character on the read-out wheel is indicated at the slot 24. Rotation of the knob 30 causes the dogs 38 of the clutch member 35 engaged in the corresponding recesses in flange 46 of the lead screw 40 to drive the lead screw and, at the same time, the dogs 52 of the spider 54 in engagement in the opposite ends of the same recesses of the flange 46 drive the spider 54 which rotates the nut 62 which in turn, through its teeth 64, rotates the engaged print-out wheel 70. The lobe-shaped projections of the corresponding print band carrier wheel 82 are rotated by engagement with the teeth 71 of the corresponding readout wheel 70 and the print band 81 indexes between the positions determined both by the dimples 1238 engaging the successive radial grooves and by the interaction of the bar 92 and the apertures 93 of the wheels 90. The position of the nut member 62 is checked by inspection through the slot 24.

If it is desired to change the operative character on one of the other print bands, the knob 30 is depressed and the dogs 38 of the clutch member 35 serve to drive the dogs 52 of the spider 54 substantially out of engagement with the recesses in the flange 46 of the lead screw and the dogs 56 at the other end of the spider into engagement with the fixed dogs 58. Rotation of the spider 54 is then effectively prevented and rotation of the knob 30 serves only to drive the lead screw 40 and, because of the screw-threaded meshing engagement of the nut member 62 with the thread 41, of the lead screw, the nut member 62 tracks until it lies opposite the read-out wheel 70 which is to be adjusted. When the nut member is correctly positioned its surface becomes visible through the slot 24. Subsequently, when inwards pressure is released, the knob 30 is moved outwardly under the action of the spring 63 so that the dogs 56 and 58 disengage and the dogs 62 re-engage in the recesses 49 of the flange 46. The rotation of the selected read-out wheel and corresponding print band can then proceed as hereinbefore described.

The shape (see FIGS. 10 and 11) of the dogs 52 of the spider is such that, when substantially disengaged, the dogs click over the edges of the recesses of the flange 46 under the action of the spring 63 and with the assistance of the profiled teeth 52 and recesses in the flange 46 with chamfered edges. The pitch of the thread 41 is so selected that the nut member 62 moves over a distance equal to the spacing between two of the narrowest print bands 81 and hence read-out wheels 70 for three clicks, a distance equal to the next wider print band for four clicks and a distance equal to the widest print band for six clicks.

It will be apparent from FIG. 18 that the shaft 86 is eccentric in relation to the sleeve 89. This arrangement facilitates mounting of the print bands 81 on the wheels 82 and 90, since the bands can be assembled loose and subsequently tightened.

I claim:

1. In an adjustable facet print mechanism, a plurality of print-facet carrying bands, each said band having a plurality of print-facets, selected ones of which are operative at any given time; a plurality of first wheels arranged on a common axis, each wheel having an internal array of projections and carrying a series of characters each corresponding to a respective print-facet of a related print band and providing an indication by display of the character corresponding to the then operative print-facet of said related print band, a plurality of second wheels arranged on a common axis parallel to the axis of the first wheels and each serving in part to support a respective print band; means supporting said bands at a position spaced from said second wheels and providing backing surfaces for the print-facets of the bands operative at any given time; means drivingly connecting corresponding first and second wheels; a screwthreaded member rotatable on a common axis of said first wheels and lying within said first wheels; stationary means rotatably supporting said screw-threaded member; a nut member carried by the screw-threaded mem' her and having an array of projections on its periphery; a rotatable spider member coaxial with said screwthreaded member and having a plurality of arms extending axially of said screw-threaded member and interdigitating with at least some of said projections of said nut member; means operable to prevent rotation of said spider member; and a single rotatable actuating member, movable axially of said screw-threaded member over a distance substantially less than the axial length of said plurality of first wheels, operative selectively, in a first axial position, to drive said screwthreaded member with the means operable to prevent rotation of said spider member operative, whereby to move said nut member to a position whereat the pro jections thereof mesh with the internal projections of a selected first wheel and, in a second axial position, to drive said screw-threaded member with the means operable to prevent rotation of said spider inoperative, whereby to rotate said nut member and spider together to index the selected first wheel and associated print band to bring a required print facet to an operative position.

2. A print mechanism according to claim 1, wherein the means serving to support said bands at a position spaced from said second wheels comprises a plurality of third wheels arranged on a common axis parallel to the axis of said first wheels, each said third wheel hav ing a polygonal periphery, the flats of which serve as said backing surfaces.

3. A print mechanism according to claim 2, wherein each said print band has a plurality of regularlyspaced internal projections and wherein each said second wheel has complementary spaced recesses engageable by the internal projections of the associated print band.

4. A print mechanism according to claim 2, wherein each said print band has a plurality of regularly-spaced internal projections and each said third wheel has corresponding recesses each disposed at mid-length of a flat of the wheel.

5. A print mechanism according to claim 1, wherein said means operable to prevent rotation of said spider member comprises a first array of dog members on said stationary means; a second array of dog members on said spider member and rotatable therewith; said first and second arrays of dog members being selectively interengageable to prevent rotation of said spider memher.

6. A print mechanism according to claim 5, in which said stationary means comprises a casing carrying said first array of dog members; said spider member being movable axially to effect engagement and disengagement of said first and second arrays of dog members.

7. A print mechanism according to claim 6, comprising clutch means controlled by said single actuating member, and serving to couple said screw-threaded member to the actuating member and to effect engagement of said first and second arrays of dog members.

8. A print mechanism according to claim 7, comprising spring means biasing said arrays of dog members to a disengaged relation.

9. A print mechanism according to claim 7, comprising overload means interposed between the actuating member and the clutch means.

10. A print mechanism according to claim 7, wherein said clutch means comprises means defining an internal polygonal surface with an even number of flat sides; said actuating member comprising a knob carrying two elongate members, each oflimited flexibility, extending transversely to the axis of rotation thereof and engaging parts of the polygonal surface, the length of each said elongate member being greater than the distance across the flat sides of the polygonal member but less than the distance between opposite corners thereof; said elongate members engaging the polygonal surface at their ends whereby to transmit rotation until, under overload conditions, the members flex to an effective length equal to the distance across the flat sides and thus slip relative to those flat sides and transmit zero torque.

l l. A print mechanism according to claim 7, wherein said clutch means comprises a disc having a plurality of apertures corresponding to the number of arms of said spider member, said disc being rigid with said screw threaded member and each aperture receiving an end portion of one arm of said spider member; and further comprising a clutch member carrying a plurality of teeth corresponding to the apertures of said disc and each partially engageable in a corresponding one of the said apertures; said second array of dog members being remote from said disc; axial movement of said single actuating member serving to engage ends of the teeth of said clutch member with ends of the arms of said spider member and thereby to engage said second array of dog members, carried by said spider member, with said first array of dog members carried by said casing.

12. A print mechanism according to claim 1, comprising shim members, one shim member being interposed between each pair of adjacent second wheels, and each shim member having regularly-spaced projections on opposite faces thereof; each of said second wheels having a plurality of regularly-spaced recesses on one face thereof and an annular groove on the other face thereof; the projections on one face of each shim member engaging in the annular groove of one adjacent second wheel and the projections on the other face of each shim member engaging in the recesses of the other adjacent second wheel; the projections on said one face of each shim member engaging in the corresponding annular groove of an adjacent second wheel serving to align the second wheels and shim members, and the projections on the other face of each shim member engaging in said recesses of an adjacent second wheel serving both to align the second wheels and shim members and angularly to locate the second wheels and their corresponding print bands. 

1. In an adjustable facet print mechanism, a plurality of printfacet carrying bands, each said band having a plurality of printfacets, selected ones of which are operative at any given time; a plurality of first wheels arranged on a common axis, each wheel having an internal array of projections and carrying a series of characters each corresponding to a respective print-facet of a related print band and providing an indication by display of the character corresponding to the then operative print-facet of said related print band; a plurality of second wheels arranged on a common axis parallel to the axis of the first wheels and each serving in part to support a respective print band; means supporting said bands at a position spaced from said second wheels and providing backing surfaces for the print-facets of the bands operative at any given time; means drivingly connecting corresponding first and second wheels; a screw-threaded member rotatable on a common axis of said first wheels and lying within said first wheels; stationary means rotatably supporting said screw-threaded member; a nut member carried by the screw-threaded member and having an array of projections on its periphery; a rotatable spider member coaxial with said screw-threaded member and having a plurality of arms extending axially of said screwthreaded member and interdigitating with at least some of said projections of said nut member; means operable to prevent rotation of said spider member; and a single rotatable actuating member, movable axially of said screw-threaded member over a distance substantially less than the axial length of said plurality of first wheels, operative selectively, in a first axial position, to drive said screw-threaded member with the means operable to prevent rotation of said spider meMber operative, whereby to move said nut member to a position whereat the projections thereof mesh with the internal projections of a selected first wheel and, in a second axial position, to drive said screw-threaded member with the means operable to prevent rotation of said spider inoperative, whereby to rotate said nut member and spider together to index the selected first wheel and associated print band to bring a required print facet to an operative position.
 2. A print mechanism according to claim 1, wherein the means serving to support said bands at a position spaced from said second wheels comprises a plurality of third wheels arranged on a common axis parallel to the axis of said first wheels, each said third wheel having a polygonal periphery, the flats of which serve as said backing surfaces.
 3. A print mechanism according to claim 2, wherein each said print band has a plurality of regularly-spaced internal projections and wherein each said second wheel has complementary spaced recesses engageable by the internal projections of the associated print band.
 4. A print mechanism according to claim 2, wherein each said print band has a plurality of regularly-spaced internal projections and each said third wheel has corresponding recesses each disposed at mid-length of a flat of the wheel.
 5. A print mechanism according to claim 1, wherein said means operable to prevent rotation of said spider member comprises a first array of dog members on said stationary means; a second array of dog members on said spider member and rotatable therewith; said first and second arrays of dog members being selectively interengageable to prevent rotation of said spider member.
 6. A print mechanism according to claim 5, in which said stationary means comprises a casing carrying said first array of dog members; said spider member being movable axially to effect engagement and disengagement of said first and second arrays of dog members.
 7. A print mechanism according to claim 6, comprising clutch means controlled by said single actuating member, and serving to couple said screw-threaded member to the actuating member and to effect engagement of said first and second arrays of dog members.
 8. A print mechanism according to claim 7, comprising spring means biasing said arrays of dog members to a disengaged relation.
 9. A print mechanism according to claim 7, comprising overload means interposed between the actuating member and the clutch means.
 10. A print mechanism according to claim 7, wherein said clutch means comprises means defining an internal polygonal surface with an even number of flat sides; said actuating member comprising a knob carrying two elongate members, each of limited flexibility, extending transversely to the axis of rotation thereof and engaging parts of the polygonal surface, the length of each said elongate member being greater than the distance across the flat sides of the polygonal member but less than the distance between opposite corners thereof; said elongate members engaging the polygonal surface at their ends whereby to transmit rotation until, under overload conditions, the members flex to an effective length equal to the distance across the flat sides and thus slip relative to those flat sides and transmit zero torque.
 11. A print mechanism according to claim 7, wherein said clutch means comprises a disc having a plurality of apertures corresponding to the number of arms of said spider member, said disc being rigid with said screw threaded member and each aperture receiving an end portion of one arm of said spider member; and further comprising a clutch member carrying a plurality of teeth corresponding to the apertures of said disc and each partially engageable in a corresponding one of the said apertures; said second array of dog members being remote from said disc; axial movement of said single actuating member serving to engage ends of the teeth of said clutch member with ends of the arms of said spiDer member and thereby to engage said second array of dog members, carried by said spider member, with said first array of dog members carried by said casing.
 12. A print mechanism according to claim 1, comprising shim members, one shim member being interposed between each pair of adjacent second wheels, and each shim member having regularly-spaced projections on opposite faces thereof; each of said second wheels having a plurality of regularly-spaced recesses on one face thereof and an annular groove on the other face thereof; the projections on one face of each shim member engaging in the annular groove of one adjacent second wheel and the projections on the other face of each shim member engaging in the recesses of the other adjacent second wheel; the projections on said one face of each shim member engaging in the corresponding annular groove of an adjacent second wheel serving to align the second wheels and shim members, and the projections on the other face of each shim member engaging in said recesses of an adjacent second wheel serving both to align the second wheels and shim members and angularly to locate the second wheels and their corresponding print bands. 